The present invention relates to a method for scrambling a television signal in a CATV system to prohibit reception of the television signal by subscribers other than those specified, and to a method and apparatus for descrambling a scrambled television signal.
In a CATV system, a subscriber contracting to receive chargeable programs pays a charge every month for every program or for every channel. It is, therefore, necessary to prohibit reception for those who have not contracted for the service and who intend to gain unauthorized access. In the CATV system, unauthorized access is prevented since a television signal to be transmitted is subject to scrambling in which synchronizing signals in the television signal are compressed with a compression ratio which is changed, for example, every 15 V (frame) period to thereby disturb a picture so that a normal picture cannot be reproduced unless descrambling processing is performed on the reception side.
FIG. 1 shows a conventional method of scrambling a television signal. In a CATV center, which is where transmission occurs, a television signal having a base band shown in FIG. 1(a) is converted into a television intermediate frequency signal, hereinafter television IF signal, as shown in FIG. 1(b). After the conversion, during a horizontal blanking interval (HBI), consisting of a front porch horizontal synchronizing period and back porch, and a vertical blanking interval (VBI), the video signal is compressed with the same compression ratio as shown in FIG. 1(c), and a key signal containing the compression ratio data of the television signal is superimposed on an audio IF signal in synchronism with the horizontal synchronizing signal of the video signal as shown in FIG. 1(d). Each key signal precedes the horizontal blanking interval by 3 to 20 .mu.sec (Tx).
In a CATV home terminal unit having a descrambling function for a television signal which has been scrambled in a manner as described above, the incoming transmitted key signal is detected, and the compression data is extracted and decoded. An expansion pulse having an amplitude based on the decoded data is generated at the time of the key signal, and the television signal compressed at the transmission side is expanded in the blanking intervals by using the expansion pulse to thereby make it possible to see a normal picture.
In the case of a television signal which has been scrambled by the foregoing scrambling method, however, it is possible for a CATV terminal unit other than a licensed one to obtain unauthorized access if descrambling is made to a certain extent.
One of the descrambling methods for unauthorized CATV access is as follows. A vertical blanking interval (VBI) is detected by a method wherein a key signal which is superimposed by AM modulation on an audio signal in every horizontal blanking interval (HBI), but which is not superimposed on the audio signal in a VBI, is passed through an integrating circuit after AM-detection, or by any other method. A video signal expanded on the basis of the detected VBI is detected to perform synchronizing signal separation to thereby obtain synchronizing signals in the VBI. A free-running timing generator operates thereafter in synchronism with the synchronizing signals. The amplitude of a pulse generated from the free-running timing generator is adjusted on the basis of the integral level of the synchronizing signal in the VBI to form an expansion pulse to carry out descrambling, so as to easily gain unauthorized picture access.